1. Field of the Invention
This invention relates to subsurface safety valves for controlling flow in wells, such as oil and gas wells, and more particularly relates to a subsurface safety valve controlled from a remote location, such as at the surface and which responds in a minimum time. More specifically, the invention relates to a remotely controllable pilot valve for a conventional subsurface safety valve operated by control fluid pressure communicated from the surface.
2. History of the Prior Art
It is well known to use subsurface safety valves for control of fluid flow such as oil and gas in a tubing string in a well bore. Such a subsurface safety valve of the wireline retrievable type is illustrated and described in U.S. Pat. No. 3,703,193 issued Nov. 21, 1972. The safety valve shown in such patent has a hydraulically operated piston for holding the valve open in response to hydraulic fluid pressure conducted to the valve through a control fluid conductor extending to the surface end of the well bore. It will be obvious that for the operator piston of such a subsurface safety valve to move upwardly for closing the valve, the piston must raise a column of control fluid equal to the distance between the subsurface safety valve and the surface end of the well bore. Substantial time can be involved in the closure of such a subsurface safety valve due to this column of control fluid. One solution to the problem of the time delay required for the subsurface safety valve to react against the column of control fluid has been the use of a pilot valve connected downhole near the subsurface safety valve between the source of control fluid pressure and the safety valve, for shutting off the control fluid pressure to the valve and releasing the control fluid pressure in the safety valve into the tubing string immediately above the safety valve, thus, eliminating the need for the safety valve piston to lift the column of control fluid between the safety valve and the surface. Such a pilot valve is illustrated and described in U.S. Pat. No. 4,119,146 issued Oct. 10, 1978. The pilot valve shown in U.S. Pat. No. 4,119,146, is hydraulically operated and responds to a change in the control fluid pressure. Thus, the response time of the pilot valve is necessarily long because of the time required for a hydraulic pressure signal change to travel from the surface to the pilot valve and because the valve must lift the column of hydraulic control fluid a short distance upwardly to move from a first lower position to a second upper position for shutting off control fluid pressure to the safety valve and releasing the safety valve control fluid pressure into the tubing string above the safety valve. Also, the pilot valve of U.S. Pat. No. 4,119,146 does not open the control fluid line to the surface into the tubing string. Often subsurface safety valves are located at depths of several thousand feet in a well bore. Thus, the time for even a pilot operated subsurface safety valve located at a depth of several thousand feet to react to a change in control fluid pressure can be substantial even in the case of a pilot valve which releases the control fluid pressure into the tubing string.